Atlantis will put NASA's fourth Tracking and Data Relay Satellite (TDRS-E) into orbit on Space Shuttle mission STS-43 to update the satellite tracking network, resulting in two operating satellites ... plus a complement of two spares in the space network.
TDRS-E, to be deployed from Atlantis about 6 hours after launch, will be boosted to a geosynchronous orbit by an attached upper stage where TDRS-E will be positioned to remain stationary 22,400 miles above the Pacific Ocean southwest of Hawaii.
The Tracking and Data Relay Satellite System, in operation since the eighth Space Shuttle flight, provides almost uninterrupted communications with Earth-orbiting shuttles and satellites and has replaced the intermittent coverage provided by globe-encircling ground tracking stations used during the early space program. A reduced string of ground stations remains in operation, however, for radar tracking and backup communications.
[Summary provided by NASA]